A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way than merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuverable than merchant ships. Unlike a merchant ship, a warship typically only carries weapons, ammunition and supplies for its own crew (rather than merchant cargo). Warships usually belong to a navy, though they have sometimes been operated by individuals or companies.

In wartime, the distinction between warships and merchant ships is often blurred. In war, merchant ships are often armed and used as auxiliary warships, such as the Q-ships of the First World War and the armed merchant cruisers of the Second World War. Until the 17th century it was common for merchant ships to be pressed into naval service and not unusual for more than half a fleet to be composed of merchant ships. Until the threat of piracy subsided in the 19th century, it was normal practice to arm larger merchant ships such as Galleons. Warships have also often been used as troop carriers or supply ships, such as by the French Navy in the 18th century or the Japanese Navy during the Second World War.

Evolution of warships

The age of galleys

In the time of Ancient Persia, Ancient Greece and the Roman Empire, the most common type of warship was the galley (such as biremes, triremes and quinqueremes), a long, narrow vessel powered by banks of oarsmen and designed to ram and sink enemy vessels, or come alongside the enemy so its occupants could be attacked hand-to-hand. However with the development of catapults in the 4th century BC and the subsequent refinement of its technology enabled the first fleets of artillery equipped warships by the Hellenistic age. With the political unification of the Mediterranean Sea in the 2nd and 1st centuries BC, naval artillery fell out of use.

Throughout late antiquity and the Middle Ages until the 16th century, naval warfare relied on the ship itself, used as a ram, the swords of the crew, and various missiles such as bows and arrows and bolts from heavy crossbows fixed on a ship's bulwarks. Naval warfare primarily involved ramming and boarding actions, so warships did not need to be particularly specialized.

Diagrams of first and third rate warships, England, 1728

The age of sail

Naval artillery was redeveloped in the 14th century, but cannon did not become common at sea until the guns were capable of being reloaded quickly enough to be reused in the same battle. The size of a ship required to carry a large number of cannons made oar-based propulsion impossible, and warships came to rely primarily on sails. The sailing man-of-war emerged during the 16th century.

By the middle of the 17th century, warships were carrying increasing numbers of cannon on their broadsides and tactics evolved to bring each ship's firepower to bear in a line of battle. The man-of-war now evolved into the ship of the line. In the 18th century, the frigate and sloop-of-war – too small to stand in the line of battle – evolved to convoy trade, scout for enemy ships and blockade enemy coasts.

Steel, steam and shellfire

During the 19th century a revolution took place in the means of propulsion, armament and construction of warships. Steam engines were introduced, at first as an auxiliary force, in the second quarter of the 19th century.

The Crimean War gave a great stimulus to the development of guns. The introduction of explosive shells soon led to the introduction of iron, and later steel, armour for the sides and decks of larger warships. The first ironclad warships, the French Gloire and British Warrior, made wooden vessels obsolete. Metal soon entirely replaced wood as the main material for warship construction.

From the 1850s, the sailing ships of the line were replaced by steam-powered battleships, while the sailing frigates were replaced by steam-powered cruisers. The armament of warships also changed with the invention of the rotating barbettes and turrets, which allowed the guns to be aimed independently of the direction of the ship and allowed a smaller number of larger guns to be carried.

The final innovation during the 19th century was the development of the torpedo and development of the torpedo boat. Small, fast torpedo boats seemed to offer an alternative to building expensive fleets of battleships.

The Dreadnought era

Another revolution in warship design began shortly after the turn of the century, when Britain launched the all-big-gun battleship Dreadnought in 1906. Powered by steam turbines, she was bigger, faster and more heavily gunned than all existing battleships, which she immediately rendered obsolete. She was rapidly followed by similar ships in other countries.

Britain also developed the first battlecruisers. Mounting the same heavy guns as the Dreadnoughts on an even larger hull, battlecruisers sacrificed armour protection for speed. Battlecruisers were faster and more powerful than all existing cruisers, which they made obsolete, but battlecruisers proved to be much more vulnerable than contemporary battleships.

The torpedo-boat destroyer was developed at the same time as the Dreadnoughts. Bigger, faster and more heavily gunned than the torpedo boat, the destroyer evolved to protect the capital ships from the menace of the torpedo boat.

Second World War

During the lead-up to the Second World War, Germany and Great Britain once again emerged as the two dominant Atlantic sea powers. Germany, under the Treaty of Versailles, had its navy limited to only a few minor surface ships. But clever names, such as "pocket battleships" deceived the British and French commands. They were rudely surprised when ships such as the Admiral Graf Spee, Scharnhorst, and Gneisenau constantly raided the Allied supply lines. The greatest threat though, was the introduction of the Kriegsmarine's most lethal weapons, the Bismarck and Tirpitz. The Bismarck was sunk in a wild, short series of sea battles in the north Atlantic, while the Tirpitz caused a bit of a stir before being knocked out by the RAF. The Royal Navy gained dominance of the European theatre by 1943.

The Second World War brought massive changes in the design and role of several types of warships. For the first time, the aircraft carrier became the clear choice to serve as the main capital ship within a naval task force. World War 2 was one of the only wars in history in which several battles occurred between groups of carriers. World War 2 saw the first use of radar in combat. It brought the first naval battle in which the ships of both sides never engaged in direct combat, instead sending aircraft to make the attacks, in the Battle of Coral Sea.

Development of the aircraft carrier

A major shift in naval warfare occurred with the introduction of the aircraft carrier. First at Taranto and then at Pearl Harbor, the aircraft carrier demonstrated its ability to strike decisively at enemy ships out of sight and range of surface vessels. By the end of the Second World War, the carrier had become the dominant warship.

Modern warships

Modern warships are generally divided into seven main categories, which are: aircraft carriers, cruisers, destroyers, frigates, corvettes, submarines and amphibious assault ships. Battleships encompass an eighth category, but are not in current service with any navy in the world. Only the deactivated American Iowa-class battleships still exist as potential combatants, and battleships in general are unlikely to re-emerge as a ship class without redefinition. The destroyer is generally regarded as the dominant surface-combat vessel of most modern blue water navies. However, it must be noted that the once distinct roles and appearances of cruisers, destroyers, frigates, and corvettes have blurred. Most vessels have come to be armed with a mix of anti-surface, anti-submarine and anti-aircraft weapons. Class designations no longer reliably indicate a displacement hierarchy, and the size of all vessel types has grown beyond the definitions used earlier in the 20th century. Another key differentiation between older and modern vessels is that all modern warships are "soft", without the thick armor and bulging anti-torpedo protection of WWII and older designs.

Cruiser - A fast independent warship. Traditionally, cruisers were the smallest warships capable of independent action. Now virtually disappeared from the oceans, along with battleships and battlecruisers.

Destroyer - A fast and highly maneuverable warship, traditionally incapable of independent action (originally developed to counter the threat of torpedo boats) but now the largest independent warship generally seen on the ocean.

Dreadnought - An early 20th century battleship, which set the pattern for all subsequent battleship construction.

Fireship - A vessel of any sort, set on fire and sent into an anchorage with the aim of causing consternation and destruction. The idea is generally that of forcing an enemy fleet to put to sea in a confused, therefore vulnerable state.

Submarine - A ship capable of remaining underwater for extended periods of time. Submarines in the world wars could stay under for less than a day, but development of nuclear reactors and air-independent propulsion allows submarines to stay submerged for weeks, even months at a time.

Modern warships come in the same "class" of ship as they did in World War II, but the sizes are different. From largest to smallest:

Aircraft carriers are often called "floating cities" because they can hold hundreds of planes, and thousands of sailors.

Escort carriers are smaller, sometimes older aircraft carriers, that hold less planes and sailors, but are usually faster. One kind of escort carrier is used by the marines to launch lots of helicopters, and is called an LHD.

Battleships, which are not often used anymore by most countries because they are expensive to operate and hard to keep safe from enemy attacks. Very few of these exist anymore, and those that do are all in museums. Smaller battleships may be called battlecruisers. Battlecruisers have less armor and guns than a battleship, but are a bit faster.

Cruisers and Frigates are often now the same size. They are mainly used to launch guided missiles, or to shoot down enemy planes. Cruisers traditionally are slightly slower and larger, and have longer range than frigates, which are slightly smaller and faster. Most cruisers and frigates are large enough to hold helicopters on the back of the ship. This part is called the fantail. Cruisers can usually hold one or two, while some frigates often can hold two or three, even though they are smaller, because of their special design.

Destroyers are smaller and faster than cruisers and frigates, and are mainly used to hunt down enemy submarines. Sometimes they will also attack enemy planes. They can also go closer to shore than larger ships, so sometimes they will attack things on land.

Other warships are used to supply ammunition, food, and other supplies to the larger warships. Some warships exists because they have powerful computers that can spy on nearby ships. Usually these kinds of ships are smaller, and do not have many guns or missiles themselves.